This invention relates to photomultiplier tubes, and, more particularly, to the processing thereof during original manufacture.
A photomultiplier tube (PMT) is an extremely sensitive detector of light and other radiation and has many uses in commercial applications. The PMT is in the form of a glass vacuum tube in which electrons released by radiation and striking a photocathode in the tube are accelerated for greatly amplifying the signal obtained from small quantities of radiation.
The photocathode of the PMT is located at its distal end, with the opposite proximal end of the photocathode having exposed ends of multiple electrical leads which extend into the evacuated tube. Also disposed inside the tube is an anode which cooperates with the photocathode. Focusing electrodes and many dynodes within the tube also cooperate with the photocathode. The dynodes are electrodes specifically configured for emission of secondary electrons which multiply the photoelectrons emitted from the photocathode upon bombardment with incident light or radiation.
The various internal components of the PMT are suitably supported on a ceramic substrate for example, with the electrical leads extending through a significant portion of the length of the tube from the respective dynodes and other electrodes through the proximal end of the tube for being connected into the corresponding electrical device in which they may be used.
For example, one application of the PMT is in a positron emission tomography (PET) scanner for medical diagnostic applications, such as detecting tumors. Many other applications of PMTs are found in scientific, medical, and commercial endeavors.
PMTs are manufactured by assembling together the internal components, inserting this subassembly core into the enclosing glass tube, and then permanently sealing the glass tube shut by fusing the glass tube to the glass base through which the electrical leads extend. The glass base typically includes a center tube through which the assembled tube is evacuated to a sufficient level of vacuum, with the tube then being sealed shut.
The PMT is then inspected for any defects which would render it inoperable for its intended use. For example, during initial manufacture of the subassembly core the individual dynodes are typically resistance-welded to the ends of the corresponding electrical leads. In the process of resistance welding, fine whiskers of weld material may be formed at the welded joints. These metal whiskers may break during the process of inserting the core into the enclosing glass tube, and be liberated in the tube.
Furthermore, small particles of ceramic substrate or other materials of the PMT core may also be liberated during the manufacturing process. Fabric fibers and hair from manufacturing workers may even be found inside the evacuated tubes notwithstanding normal precautions taken during the manufacturing process for ensuring clean assembly of the various components.
Since the PMT is evacuated and operated at high voltage during use, these loose debris particles or contaminants may adversely affect the performance of the PMT or substantially reduce its useful life.
Inspection of the manufactured tubes may reveal relatively large particles that are equal to or greater than 0.2 mm in length and thus deemed unacceptable for meeting the desired performance and life specifications for the PMT. In particular, liberation of the weld metal whiskers is a significant concern, and improved welding techniques are being investigated for reducing the occurrence of this one type of particle in the manufactured PMT.
Weld metal whiskers constitute the majority of undesirable debris particles typically found in the manufacture of PMTs, and their reduction or elimination can substantially reduce the defect rate. Since the glass tube of the PMT is fused shut under vacuum during the manufacturing process, it is impossible to physically remove any undesirable debris therein, and defective PMTs must then be scrapped at a corresponding substantial cost.
Accordingly, it is desired to provide a method of processing PMTs during the manufacturing process for reducing defects due to debris particles in the PMTs.
A photomultiplier tube includes dynodes electrically joined to corresponding leads. The tube may be reprocessed by positioning a liberated debris particle at an accessible site inside the tube. A power laser is aimed at the particle through the wall of the tube and fired for reducing the size of the particle.